Laser Ablation Mass Removal versus Incident Power Density during Solid Sampling for Inductively Coupled Plasma Atomic Emission Spectroscopy

For laser ablation solid sampling, the quantity of material ablated (removed) influences the sensitivity of chemical analysis. The mass removal rate depends strongly on the laser power density, which is the main controllable parameter for a given material and wavelength parameter using laser solid sampling for inductively coupled plasma atomic emission spectroscopy (ICP-AES). For a wide variety of materials, a decrease in the rate of change, or roll-off, in mass removed is observed with increasing incident laser power density. The roll-off results from a change in the efficiency of material removed by the laser beam, primarily due to shielding of the target from the incident laser energy by a laser-vapor plume interaction. Several analytical technologies were employed to study the quantity of mass removed versus laser power density. Data for mass ablation behavior versus laser power density are reported using ICP-AES, atomic emission from a laser-induced plasma near the sample surface, acoustic stress power in the target, and measurements of crater volumes. This research demonstrates that the change in ICP-AES intensity with laser power density is due to changes in the mass removal. The roll-off in mass ablation is not due to a change in particle size distribution of the ablated species, fractionation of the sample, or a change in transport efficiency to the ICP torch. Accurate tracking of the ICP-AES with the laser ablation process justifies the use of internal and external standardization.